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The Problem with Paralleled NTC Thermistors as Inrush Current Limiters And an Update With a Alternate Device Compiled and Maintained by Mike Morris WA6ILQ |
A number of the Astron supplies have inrush reducing Negative Temperature Coefficient (NTC) thermistors and unfortunately some of those use a pair in parallel. For example, the early SS-25 and SS-30 use a pair of 4 ohm 5 amp units (identified as RT1 and RT2 on the schematic). Look here at the top left. (opens in a new browser tab)
(Anybody have an SS-25 or an SS-30 that they can get a cellphone photo of the PC board showing the RT1 and RT2 positions?)
You can not parallel NTC thermistors to get additional current rating. They might have the same part number and the same specifications but the thermistors are not identical (some are even 15% tolerance) and won't evenly divide the current. It won't be 50% / 50% no matter how much wishful thinking is involved. One of them will heat up before the other, and display a lower resistance and pass more current. The other will be a higher resistance, and therefore carry less current. Eventually one will burn up then the other unit will be forced to carry 100% and it too will burn up / out and your supply will be dead. As a quick field fix just replace either one with a jumper wire.
If you are going to use NTC thermistors as inrush reducers you need to use a single device. In the case of the Astron SS-25 or SS-30 it needs to be rated at 8 to 10 amps or so. As a repeater site emergency fix just replace it with a piece of wire (there's a fuse on the input to the supply, it's not going to burn up). When you get it back to the shop replace the pair with a single device.
The newer Astron supplies use a Bourns BN-LG15Y0R7MYB (it is rated at 0.7 ohms at 25 °C and is good for 8 amps) or use the SCK0R78A (also known as SCK150R78AMSY) thermistor. Digi-Key has the Bourns part in stock as BN-LG15Y0R7MYB. (off-site link, opens in a new browser tab)
Update Febuary 2026 by Mike Morris WA6ILQ:
Jeff Banke NZ2S posted on a mailing list a PDF from Amphenol-Thermometrics titled "
NTC Inrush". (off-site pointer, opens in a new browser tab)
The writeup includes a full product selection procedure and a product line data sheet.
At the time of this update the devices are stocked by DigiKey, Mouser and Newark.
Just look at the table below, note the max AC current draw, then look at the data sheet.
Here's a table of the Astron supplies at the time of this update. The second column is the AC line current when the DC side is fully loaded.
| Model(s) | Max AC current (amps) |
|---|---|
| RS-4 | 0.7 |
| RS-5 | 0.92 |
| RS-7 | 1.2 |
| RS-12 / RM-12 / VS-12 | 2.0 |
| RS-20 / RM-20 / VS-20 | 3.4 |
| RS-35 / RM-35 / VS-35 | 5.7 |
| RS-50 / RM-50 / VS-50 | 8.2 |
| RM-60 | 9.14 |
| RS-70 / VS-70 | 11.4 |
| SL-11 | 2.0 |
| SL-15 | 2.25 |
| VS-12 | 2 |
| SS-10 | 1.3 |
| SS-12 | 1.6 |
| SS-18 | 2.1 |
| SS-25 | 3.6 |
| SS-30 | 4.5 |
| SS-50 | 7.2 |
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This page was created January 2026 by WA6ILQ.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.